An uninterruptible power supply system has conventionally widely been used as a power supply device for supplying AC power to an important load such as a computer system in a stable manner. For example, as shown in Japanese Patent Laying-Open No. 2006-109603 (Patent Document 1), an uninterruptible power supply system generally includes a converter for converting AC power to DC power, and an inverter for converting DC power to AC power. Usually, the converter converts AC power from a commercial AC power supply to DC power, and supplies the DC power to the inverter while charging a power storage device such as a storage battery. The inverter converts DC power to AC power and supplies the AC power to the load. When the commercial AC power supply fails, power from the power storage device such as a storage battery is supplied to the inverter, which continues to supply AC power to the load.
In addition, a power conversion device including a converter for converting AC power from a commercial AC power supply to DC power and an inverter for converting that DC power to AC power having desired frequency and voltage is also available. For example, Japanese Patent Laying-Open No. 2003-070262 (Patent Document 2) discloses a power conversion device including a three-level PWM converter and a three-level PWM inverter. In this power conversion device, the three-level PWM converter includes four switches connected in series and a smoothing capacitor, and the three-level PWM inverter includes four switches connected in series. In addition, in order to prevent breakage of a highly frequently used switch among eight switches due to heat generation, the highly frequently used switch is constituted of two semiconductor switching elements connected in parallel.    Patent Document 1: Japanese Patent Laying-Open No. 2006-109603    Patent Document 2: Japanese Patent Laying-Open No. 2003-070262